The present invention relates to a spring unit for use in a mattress or a box spring.
A conventional spring unit used in a mattress or a box spring comprises a plurality of first main springs and a plurality of second main springs shorter than the first ones. Each main spring is made of one wire and has a straight rod portion and two spring portions. The spring portions are coupled at the upper end to the ends of the rod portion, respectively. The main springs are arranged on a rectangular base plate. More precisely, the first main springs are arranged parallel to the long sides of the base plate, whereas the second main springs are arranged parallel to the short sides of the base plate. Hence, the first and second main springs form a lattice. The lower ends of the spring portions of each spring are secured to the four edges of the base plate. The rod portions of the main springs are supported by intermediate support springs. Like the main spring, each intermediate support spring has a straight rod portion and two spring portions. The spring portions are coupled at the upper end to the ends of the rod portion, and at lower end to the base plate.
In the known spring unit, the spring portions of the main springs and intermediate support springs are torsion bar springs. A torsion bar spring deforms very slightly when a compressive load is applied to it. But, it will be permanently deformed if compressed many times. The spring unit, which comprises torsion bar springs, may lose its elasticity in a relatively short time.